This disclosure relates to acidified polyamidoamine adhesive compositions, methods for their manufacture, and their use in creping and ply bonding applications.
In the manufacture of certain wet-laid paper products, such as facial tissue, bathroom tissue, napkins, or paper towels, the web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness stretch and bulk. The creping process involves adhering the web to a rotating creping cylinder, such as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker causing a formation of microfolds “crepe bars”.
The severity of this creping action is dependent upon a number of factors, including the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion increases softness, although generally with some loss of strength. In order to increase adhesion, an adhesive creping aid is used to enhance any naturally occurring adhesion that the web may have due to its water-content. Water content can vary widely, depending on the extent to which the web has been previously dried. Adhesive creping aids prevent wear of the dryer surface, provide lubrication between the doctor blade and the dryer surfaces, reduce chemical corrosion, and control the extent of creping. The creping process can be wet or dry.
In either the wet or dry crepe manufacturing process, crepe adhesives can be either “soft” or “hard” polymers. Soft polymers are typically non-crosslinked polymers. Two known soft polymer adhesives are polyamines and polyvinyl alcohol (PVOH). Examples include BBD 2071, a polyamine commercially available from Buckman Laboratories International, Inc., and Airvol®, a polyvinyl alcohol sold by Air Products and Chemicals, Inc. Both materials are non-crosslinked and provide high adhesion, and are considered highly moisture sensitive. These are used to fabricate low sheet moisture grade tissues such as bath tissue on TAD (Thru Air Dryer) processes. Premium quality bath and facial tissue (<4.0% sheet moisture) are fabricated on dry crepe processes. Disadvantages of these adhesives are that they dissolve easily (slight to no insolubility); they tend to associate with the sheet and the moisture in the sheet because they are not crosslinked; they have an uneven dryer coating profile; they have fair to poor Yankee dryer and blade protection (low add-on due to high adhesion); and some are corrosive, including PVOH.
Hard polymers are typically crosslinked. The level of crosslinking in commercial hard creping adhesive products ranges from slight to high. Useful hard creping aids include epi-polyamides, which are thermoset resins (i.e. polymers that lose solubility and fusibility upon crosslinking). They provide an adhesive coating durable enough to withstand the mechanical forces at the doctor blade under conditions of heat and/or moisture. Epi-polyamides are derived from the crosslinking of polyamidoamines with epihalohydrins. Polyamidoamines themselves are condensation polymerization products formed by the reaction of a dicarboxylic acid and a polyamine having at least two primary amines and either a secondary or tertiary amine. Polyamidoamines are manufactured in an alkaline environment (pH 8.0-10.0). The pH is typically reduced to 3.0-5.0 prior to or after the addition of the epihalohydrin. Maintaining a low pH slows the crosslinking reaction and maintains product viscosity. As the pH is increased the product crosslinks, viscosity significantly increases and the product can become unstable, resulting in gellation.
While suitable for their intended purposes, crosslinked adhesives also have certain disadvantages. They tend to build up on the dryer surface, which pushes the blade back and causes sheet picking. They are ineffective on virgin fiber grades (low sheet moisture) that require high adhesion and better rewet. They also accumulate on dryer edges (the hottest section of the dryer), causing uneven profile and/or sheet breaks. The harder the base coating, the more that can build up and increase the z-directional coating thickness. Ideally, a balanced equilibrium should be established that the crepe chemical add-on, sheet fines and filler that come from the sheet equals the amount of coating that is scraped and washed off the dryer surface.
With very high coating hardness (typically on the dryer surface edges where the dryer surface temperatures is the highest), a bevel blade (5, 10 or 15° angle) at increased blade holder pressure is more effective in scraping off the buildup. The use of a bevel blade on bath tissue grades, however, can also have negative effects (such as lower bulk, bare spots on the dryer and excessive blade and dryer wear. Changing the bevel angle and increasing pressure can result in low sheet bulk, requiring lower sheet moisture of 3.0 to 3.5% (much lower then desired) in order to recover some bulk. Lower sheet moisture can require higher Yankee steam (higher dryer surface temperature) exacerbating the problem by further hardening the adhesive.
Some tissue machines utilize a separate cleaning blade located after the doctor blade and before the crepe spray boom system. The cleaning blade is typically set at a pressure from 69 to 276 megapascals (MPa) (10 to 40 pounds per square inch (psi)) and designed to remove the excess coating buildup peaks, providing improved overall surface coating profile.
Limitations also arise in connection with use of the Yankee dryer crepe spray boom configuration. First, the newly applied crepe adhesive starts crosslinking and setting on the Yankee dryer surface shortly after emerging from the spray boom. The setting rate of the adhesive depends on the dryer type, temperature, speed, sheet moisture, and pH. Sometimes, debunizers (steam box prior to the vacuum pressure roll) or infrared heater are used for additional sheet drying and can affect the coating drying rate. In most processes, there is no available space to move the spray boom closer to the blade holder to improve the adhesive's setting time. If an insufficient coating base is present, an adhesive that has a higher degree of crosslinking is used.
Second, the spray boom dilution water rewets the existing coating base that remained on the dryer. As the coating base swells and softens, the adhesive is retackified, allowing the sheet to properly transfer from the vacuum pressure roll to the dryer surface. There are occasions when a heavily crosslinked adhesive can not be sufficiently rewetted and softened and the Z directional coating thickness builds up causing operational problems.
Other drawbacks are associated with water-soluble, thermosetting cationic epi-polyamide resins. The physical properties of these resins, such as insolubility, rewet, adhesion, and hardness, are in large measure controlled by the degree of cross-linking by the epihalohydrin. It is very difficult to vary these properties for a given creping machine. Paper makers have recognized that the different creping machines and different wood pulps have different properties with the contemporaneous need for changing the properties of the creping adhesive to optimize the creping process. Paper makers have long wanted the ability to control and change the amount of cross-linking to achieve the precise performance characteristics desired for a given papermaking line to optimize the creping process. With the water-soluble, thermosetting cationic epi-polyamide resins of the prior art, that degree of control has been difficult to achieve because the cross-linking of the adhesive occurs during the manufacturing process and it is impractical to have a number of water-soluble, thermosetting cationic epi-polyamide resins with different degrees of cross-linking in an attempt to tailor the creping adhesive to the creping process.
Other adhesive deficiencies relate to plied tissue products. Laminating multiple layers, or plies, of tissue, makes various tissue products. The plies must be bonded to prevent them from delaminating or floating apart when they are converted into the final product or when they are used by the consumer.
Ply bond properties vary with paper grades and machine conditions (i.e., double layer sheets using two head boxes, multi-cylinder machines making board and napkin/towel bonding of two or more plies). Starch and latex polymer are typically used as ply bonding agents. Starch is unstable and typically requires cooking and make down dilution, and exhibits sporadic performance. Latex can have repulpability and nozzle plugging issues.
Ply bonding of the individual tissue sheets into multi-ply layers is usually achieved by embossing them on converting equipment without applying adhesive. In this process, two or more tissue webs are simultaneously unwound and fed through a nip formed between male and female embossing rolls to emboss or crimp the webs and thereby bond them together. Often in making such products as napkins, the webs are embossed only around the perimeter of areas that will be cut into the individual napkins. In a different process, tissue plies are adhered using a chemical adhesive rather than by embossing.
Controlling ply bonding is important and difficult. Inadequate, excessive, or inconsistent ply bonding can jam complex, high-speed machinery, generate high waste, and provide unacceptable product. The strength of bonding by embossing (without adhesive) may vary depending, among other things, on water content or dryness of tissue webs, and on ambient air humidity. While bonding by embossing sometimes can be improved by increasing the pressure on the embossing rolls, such pressure can wear out the embossing rolls more quickly, particularly the female roll, which is usually a softer roll made of composite material onto which a pattern is impressed by the opposing, male, engraved metal roll. In addition, the journals and bearings of both embossing rolls can also wear out prematurely if subjected to increased pressure over a prolonged period.
Creping adhesives overcome some of the problems of embossing, but they also cause problems, such as “through bonding” or “blocking”, in which adjacent laminates bond to one another, and, prevent unwinding of the laminate product from its roll. Non-uniformity of bonding also causes problems, such as wrinkling of the tissue and bad printing.
Another product requirement in ply bonding is wet strength, the resistance to tearing or pulling apart while wet, which can be critically important to high quality paper napkins. Low wet strength napkins may pull apart when subjected to moisture, such as spilled liquid or a wet glass, rendering them unsatisfactory for the high quality markets. Chemicals normally employed to impart wet strength (e.g., urea, phenol-formaldehyde) can reduce ply bond-strength and can make the napkin stiffer.
Floating plies at the printing stage cause jams, high waste and machine downtime. In addition, the application of the ink causes the top layer of the tissue to expand if the plies are not well bonded, resulting in wrinkling, badly printed napkins, and extremely high costs from waste.
Commonly used ply bonding adhesives are aqueous mixtures, for example, carboxymethyl cellulose, polyvinyl alcohol, or starch. The spraying process control described above enables the use of higher (for example at least 13%) solids in the liquid adhesive, which in turn results in shorter drying times. After the adhesive is applied, the webs may be forced together by a pair of rolls (e.g. calendar rolls) that are in contact as the two webs pass through.
While a number of creping and ply bonding adhesives have been disclosed and are suitable for their intended uses, no single adhesive or adhesive blend has provided a satisfactory combination of adhesive performance characteristics (i.e. insolubility, rewet, adhesion and hardness) on different creping machines. Thus, a continuing need exists for creping and ply bonding adhesives that overcome the above deficiencies and limitations.